1. Field of the Invention
This invention relates to measuring instruments including a micrometer or the like, and more particularly to improvements in spindle driving construction for moving spindles of the measuring instruments.
2. Description of the Prior Art
There have been proposed by the present applicant micrometers, in each of which a spindle is linearly and axially movable, while a screw feed system is adopted, but yet, the spindle is not rotated (Japanese Patent Kokai (Laid-Open) Nos. 179701/82, 15101/83 and Japanese Utility Model Application No. 185116/81). The micrometers thus proposed, being capable of driving the spindle at high speed and easy in holding the measuring pressure, are excellent in the controllability at high speed and the easiness in handling.
Now, in each of the proposed micrometers, spindle rotation locking means has been constituted by an engageable member projected in the radial direction of the spindle and a tubular guide member having an elongate groove, to which the engageable member is coupled, and the elongate groove has been opened at one end of the guide member. The opening at one end of the elongate groove has been due to the reason for assembling, but, due to the reason for finishing. More specifically, the guide member must be a tubular member having a comparatively large wall thickness because it holds the spindle, while, in order to satisfy the finishing accuracy of the elongate groove, the use of an end mill or the like is not satisfactory and grinding works are required. Therefore, if one end of the elongate groove would be blocked, then the length of the elongate groove would not be sufficient. For example, FIGS. 1A and 1B show a tubular member 1 of a small wall thickness and a tubular member 2 of a large wall thickness, both of which have outer diameters equal to each other. It is apparent that, when the both tubular members are ground by one and the same grinding machine 5 to form elongate grooves 3 and 4, if the elongate grooves are ground in a state where opposite ends are blocked, then a length L.sub. 2 of the elongate groove 4 of the tubular member 2 of the large wall thickness is considerably short as compared with a length L.sub.1 of the elongate groove 3 of the tubular member 1 of the small wall thickness.
Then, in each of the proposed micrometers described above, in order to secure the wall thickness of the guide member to a certain degree, but yet, make the length of the elongate groove sufficient, the elongate groove has been opened at one end of the guide member. However, in such a case as above, the following disadvantages tend to occur.
Since the elongate groove is provided for coming into contact with an engageable member projectingly provided on the spindle to lock the rotation of the spindle, the engageable member of the spindle constantly applies a force to the elongate groove to expand the elongate groove during the movement of the spindle in the axial direction, and, due to this force, such a phenomenon tends to occur that the guide member is expanded in its diameter and so on. Due to this phenomenon, a deflection (an inaccurate displacement), being slight though, occurs in the spindle. Because of this, for example, in the case where an optical detector is adopted, the positional relationship between a main scale secured to the spindle and an index scale secured to a body frame fluctuates, and after all, the adoption of a high accuracy displacement detector should not necessarily secure a satisfactorily high accuracy measurement.
Now, in the micrometer illustrated in Utility Model Application No. 185116/81 out of the above-described micrometers, a guide member being in parallel to the spindle is secured to the spindle through a connecting rod so as to maintain and regulate the spindle in its rotating direction, and the aforesaid main scale is affixed to this connecting rod, so that the spindle can be prevented from being deflected, thereby possibly enabling to achieve the high accuracy measurement. However, in the micrometer of the type described, since it is necessary to provide all of the guide member, the connecting rod and a relief space for the main scale in the interior of a main body, the measuring instrument as a whole is rendered large and complicated in construction and becomes expensive in manufacturing cost, a carrying means for preventing the vibrations and the like of the main scale must be firm in construction, and the problem of the expansion in diameter of the elongate groove by the engageable member remains unsolved.
Additionally, the above-described problem is common not only in the field of the micrometers but also in the field of the instruments having a driving mechanism for linearly moving the spindle under a screw feed system in general.